1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling capability information of a mobile device in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, supporting the third generation partnership project (3GPP) Rel-8 standard or the 3GPP Rel-9 standard which are developed by the 3GPP, is now being regarded as a wireless communication system combining a new radio interface and a radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) for communicating with a plurality of user equipments (UEs) and communicates with a core network including a mobility management entity (MME), serving gateway, etc for NAS (Non Access Stratum) control.
Before a UE starts to exchange wireless signals with an eNB, the eNB must know capability information of the UE which indicates the best capability of the UE such that the eNB can set up system parameters for exchanging the wireless signals with the UE. Therefore, the UE is necessary to transmit the capability information to the eNB, when a communication between the UE and the eNB is established or when the UE is handed over to the eNB from another eNB. After the eNB receives the capability information, the eNB can setup the system parameters according to the capability of the UE.
For example, the UE with a better capability may be capable of receiving high data rate services. Oppositely, the UE with a poorer capability may be only capable of receiving low data rate services. According to the capability information received from the UE, the eNB can provide the high data rate services to satisfy the UE with the better capability, and the eNB can provide the low data rate services to facilitate the UE with the poorer capability to receive the services. In other words, the LTE system prevents the UE with the better capability from receiving the low data rate services by using the capability information, unless the UE notifies the eNB to do so. The LTE system also prevents the UE with the poorer capability from receiving with the high data rate services that the UE cannot process.
In detail, in the LTE system, the capability information is included in a UE-EUTRA-Capability information element (IE) which includes fields such as rf-Parameters, measParameters, ue-Category, etc. More specifically, the ue-Category field includes 5 UE categories, and each UE category defines a set of parameters such as total number of soft channel bits, maximum number of supported layers for spatial multiplexing in downlink (DL), support for 64 quadrature amplitude modulation (QAM), etc.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
The carrier aggregation (CA) is introduced in the LTE-A system by which two or more component carriers are aggregated to achieve a wider-band transmission. When the CA is realized and configured, the LTE-A system can support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 component carriers, where bandwidth of each component carrier is 20 MHz and is backward compatible with the 3GPP Rel-8 standard. The LTE-A system supports the CA for both continuous and non-continuous component carriers. The CA further increases bandwidth flexibility by aggregating the non-continuous component carriers.
The CoMP is considered for different eNBs at geographically separated locations to coordinate the transmissions and receptions of a UE in the LTE-A system. When the UE is near a coverage edge of an eNB, the UE can also receive signals from neighbor eNBs (including the original eNB) on the DL, and signals transmitted by the UE can also be received by the neighbor eNBs on the UL. Accordingly, the neighbor eNBs can cooperate to reduce the interference from signal transmissions not related to the UE (e.g. by scheduling or beamforming), to improve signal reception of the UE on the DL. The neighbor eNBs may also enhance the signal reception of the UE by transmitting the same data to the UE. On the other hand, the eNBs can combine the received signals from the UE to increase a quality of the received signals on the UL. Therefore, link performance such as data rate and throughput on both the UL and the DL can be maintained or even increased, when the UE is near the coverage edge of the eNB.
The UL MIMO technique is used to achieve higher data rate, higher spectrum efficiency and improved system capacity by enabling parallel data streams to be exchanged between the eNB and the UE. In general, the UL MIMO technique should be implemented by using multiple transmitting antennas at the UE and multiple receiving antennas the eNB. However, even though the eNB can be equipped with the multiple receiving antennas, it is difficult for the UE to be equipped with the multiple transmitting antennas due to a limited size, when the 3GPP developed the LTE system. However, it may be possible that the UE can be equipped with two or four transmitting antennas in the near future with improvement of the semiconductor industry. Therefore, the UL MIMO is adopted to improve performance of the LTE-A system.
Since the best capability of the UE is improved for the LTE-A system, additional capability information is going to be included in the UE-EUTRA-Capability IE. For example, 3 new categories are added in the ue-Category field. Therefore, when a UE supports the 3GPP Rel-10 standard and can be configured with the advanced techniques such as the CA, the CoMP and the UL MIMO, the UE transmits the capability information corresponding to the advanced techniques to an eNB supporting the 3GPP Rel-10 standard, such that the eNB can set up corresponding system parameters. In this situation, the UE and the eNB can communicate with each other by using the advanced techniques, and the high data rate services can be provided to the UE.
On the other hand, the UE supporting the 3GPP Rel-10 standard may be in a coverage of a legacy eNB supporting only the 3GPP Rel-8 standard or the 3GPP Rel-9 standard. When the UE transmits the capability information corresponding to the advanced techniques to the legacy eNB, the legacy eNB cannot recognize the capability information corresponding to the advanced techniques. In this situation, since the legacy eNB does not know the capability of the UE, the legacy eNB may be conservative and treats the UE as a UE with the worst capability. Then, the legacy eNB set up system parameters to maintain only basic transmissions, e.g., the lowest data rate transmissions. In other words, the legacy eNB and the UE communicate with each other by using the worst capability which is lower than that the legacy eNB and the UE can support. Not only the UE suffers from a performance loss, but system throughput degrades. Therefore, how to communicate with the legacy eNB by using the best capability when the UE supporting the advanced techniques is in the coverage of the legacy eNB is a topic for discussion.